This invention in general relates to displacement measurement interferometry and more particularly to displacement measurement interferometers (DMIS) that utilize polarization preserving optical systems to enhance their accuracy by reducing undesirable polarization effects.
In polarization interferometers for the measurement of linear and angular displacements, one or more orthogonally polarized beams are introduced into the interferometer and then are separated by a polarizing beam splitter for travel along reference and measurement legs. Afterwards, the beams are recombined and information about the difference in the optical path lengths they traveled is analyzed to yield displacement. In the process of traveling through the interferometer, perhaps making multiple passes, the polarized beams typically encounter retroreflectors that return them from either fixed or moveable mirrors. Such retroreflectors, which are non- polarization preserving, introduce errors in the accuracy of the measurements obtainable because they do not preserve the state of polarization of the beams to a degree consistent with the demands for high accuracy.
Such non-polarization preserving retroreflectors are well-known and operate to deflect light through 180 degrees such that an incoming beam is exactly reversed in direction, traveling as an outgoing beam parallel to the direction of propagation of the incoming direction and spatially offset with respect to it. The classical retroreflector essentially contains the intersection corner of three mutually perpendicular plane surfaces and is known as the cube corner retroreflector or sometimes the tetrahedron. Here, a ray generally undergoes 3 reflections, one from each 120.degree. sector in the process of entering and exiting the retroreflector. Ideally, the direction of the reflected ray is opposite that of the incident ray but displaced due to a reflection through the retroreflector intersection corner. From the standpoint of polarization effects, the primary problem with the classical retroreflector is that the angles the rays make with the mirror surfaces are skew. Detailed calculations using the Jones matrix formalism, along with the Fresnel reflection formulas, can be used to predict the resultant polarization for different initial polarizations and retroreflector types. From the standpoint of their use in DMI applications where small, linearly polarized beams interact with only small sub-apertures of the retroreflector, the net effect is to rotate the plane of polarization by several degrees (typically 6.degree.). This phenomenon is called Retro Induced Polarization Rotation (RIPR) and misaligns the beam polarization directions with respect to the polarization beam splitter of the interferometer which can cause large periodic errors in the measured interferometric phase. A particularly troublesome periodic or "cyclic" error which occurs in High Stability Plane Mirror Interferometers (HSPMI) produces an error with a frequency at 1/2 the Doppler shift (as well as other frequencies). It has been shown that this error, which is due to the polarization rotation properties of the retroreflector, can be extremely large and will occur regardless of the beamsplitter quality.
Consequently, it is a primary object of the present invention to provide polarization interferometers utilizing polarization preserving optical systems that provide beam deflection properties without introducing deleterious polarization effects.
It is another object of the present invention to provide displacement measurement interferometers that use polarization preserving optical systems in place of traditional cube corner retroreflectors.
Yet another object of the invention to provide polarization plane mirror interferometers that utilize polarization preserving optical systems to enhance displacement measurement accuracy.
It is still another object of the present invention to provide polarization plane mirror interferometers that utilize internally tilted surfaces for purposes of reducing the effects of undesirable ghost beams on accuracy.
It is yet another object of the present invention to provide polarization plane mirror interferometers that utilize retardation elements in one or more interferometer legs to reduce the effects of ghost images.
Yet another object of the present invention is to provide polarization preserving optical systems for use at multiple wavelengths.
It is yet another object of the present invention to provide multiple-pass (two or more) polarization plane mirror interferometers that utilize polarization preserving optical systems.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter when the following detailed description is read in connection with the accompanying drawings.